Ok, I know that a lot of spell casters follow the Pagan or Wiccan religion, but is it it possible to be an atheist spell caster? Technically, atheism is defined as a belief in no gods, but it never says anything about unnatural forces. Does anyone have any opinions on this?

Yes, after a manner of speaking. I don't particularly believe in or worship any deities, seeing them more as spirits possessed of greater awareness or otherwise existing on a higher vibration, or whatever terms you want to use... But just spirits, all the same. That being said though I'm more agnostic than atheist in the strictest sense, I suppose.

I am an Atheist, and like all Atheists, I do understand the natural forces! We understand how the Earth works; how the Universe works.(Only to some extent; the Universe is still being explored!). What we do not believe are the "un-natural" forces!

Yes athiests can use magic, but they may have different ideas about what magic is. One idea is that magic is changing things, either natural or caused by humans.

Some Athiests are materialists, but these words do not mean the same thing. Materialists are people who believe the universe is only what science and our immediate sences have shown to exist and they dismiss ideas about metaphysics. If by spells, you mean sympathetic magic(as above so below) and affecting the world with will power alone, these fall under metaphysics.

It is a unfortunate that many people new to magic assume religion is a requirement. New age authors who erroneously say "witchcraft is an earth based religion" and similar things are in part to blame for this confusion, which misleads people new to magic into thinking following a path is compulsory.

Not only do modern "New Age" authors mistake witchcraft for a religion! They often write of witchcraft as "magic spells".
A spell is not magic, of itself. It is a ritual attempt to make magic happen! I was taught witchcraft by a witch who had never cast a spell in her life! I have been a witch for well over seventy years; and I have never cast a spell!

To be fair, this site has a lot of religious people and we tend to focus on "paths" and spiritual experience more than magical practice. It's all about the environment; go to another place where the environment is different and you'll encounter different attitudes and leanings.

That being said, there are plenty of secular practitioners. You just have to know what to look for and you'll find what you need.